Two enzymes from human cells that act specifically on damaged DNA will be further purified and characterized. The first is an endonuclease that acts on UV- or x-irradiated DNA. The second is an N-glycosidase that acts on methylated DNA. The levels of these enzymes will be examined in mutant human cell lines that are characterized both by unusual sensitivity to radiation or alkylation and by elevated incidences of neoplasia. These enzymes will also be used as probes to identify and measure DNA lesions induced by genotoxic agents in normal and mutant human cell lines, with a view to determining relative efficiencies of repair in such cell lines.